1. Field of the Invention
This invention relates generally to the field of perpendicular magnetic recording heads and more particularly, to recordings heads having antiferromagnetic coupling (AFC) to achieve both low remnant magnetization and low field required to reach the maximum magnetization allowing higher density recording with accuracy.
2. Description of the Prior Art
As the recording density of magnetic hard drives (or disc drives) increases, a physical limitation is experienced using longitudinal recording systems partly due to thermal relaxation known as super-paramagnetism. That is, the density requirements for meeting today's storage needs are simply not attainable with longitudinal recording systems. To provide further insight into this problem, it is anticipated that longitudinal recording systems will lose popularity as storage capacities in excess of about 150 Gigabytes-per-square-inches become a requirement. These and other factors have lead to the development and expected launch of perpendicular recording heads or write heads. Perpendicular recording is promising in pushing the recording density beyond the limit of longitudinal recording.
Accordingly, perpendicular recording potentially can support much higher linear density than longitudinal recording due to lower demagnetizing fields in recorded bits, which diminish when linear density increases.
A magnetic recording head for perpendicular writing generally includes two portions, a write head portion or head for writing or programming magnetically-encoded information on a magnetic media or disc and a reader portion for reading or retrieving the stored information from the media.
The write head or recording head of the magnetic head or disc drive for perpendicular recording typically includes a main pole and a return pole which are magnetically separated from each other at an air bearing surface (ABS) of the writer by a nonmagnetic gap layer, and which are magnetically connected to each other at a region distal from the ABS at a back gap closure (yoke). This structure is a single-pole writes head because while a main pole and return pole are referred thereto, the return pole is not physically a pole, rather, it serves to close the loop with the main pole and the soft under layer for magnetic flux circuit.
Positioned at least partially between the main and return poles are one or more layers of conductive coils encapsulated by insulation layers. The ABS is the surface of the magnetic head immediately adjacent to the perpendicular medium.
To write data to the magnetic medium, an electrical current is caused to flow through the conductive coil, thereby inducing a magnetic field across the gap between the main and return poles. By reversing the polarity of the current through the coil, the polarity of the data written to the magnetic media is also reversed.
The main and return poles are generally made of a soft magnetic material. Both of them generate magnetic field in the media during recording when the write current is applied to the coil. A magnetic moment of the main pole should be oriented along an easy axis parallel to the ABS when the main pole is in a quiescent state, namely without a write current field from the write coil. When the magnetic moment does not return to an orientation parallel to the ABS after being subjected to multiple instances of the write current field, the main pole is not stable. Therefore, in an unstable pole, the orientation of the magnetic moment might remain nonparallel to the ABS position even after current to the write coil is turned off. This is referred to as remnant moment. Accordingly, the remnant magnetic field of the main pole may deteriorate or even erase data from the disc. Further, an unstable pole results in increased switching time when current is applied. In a perpendicular head for high track density recording, the main pole is a predominant source of instability due to a strong demagnetizing field across the pole width at the ABS and the necessity of using magnetic material with high magnetic moment saturation.
Thus, the need arises for improvements to the structure of the main pole of a perpendicular recorder or write head that effectuates low remnant magnetization while requiring a low field to achieve saturation magnetization.
In the perpendicular recording heads, writing and erasing of information is performed by a single-pole write head. That is, the return pole is, in essence, a misnomer because it does not actually serve as a pole. The single-pole or the main pole is composed of high moment magnetic materials, the most common example being cobalt-iron (CoFe) alloys. One of the problems with the single-pole write head is erase-after-write or undesirable erasure of information. To avoid this problem, the high moment materials for the single-pole generally have should have very small remnant moment. Remnant moment is moment that exists in the absence of magnetic field. Remnant moment can be reduced by laminating the high moment magnetic material (for example CoFe or cobalt-iron alloy) into multilayers.
The most recent lamination scheme utilizes antiferromagnetic coupling (AFC) between neighboring CoFe (cobolt-iron alloy) laminated layers. The basic film structure is CoFe/AFC/CoFe where the two CoFe layers are laminated within a thin AFC layer (such as chromium). Due to the antiferromagnetic coupling between the two CoFe layers, the magnetic moments in the CoFe layers are in the opposite direction relative to each other so that they cancel each other at zero field, thus, realizing a very small remnant magnetization. This is the most effective way to eliminate erase-after-write.
However, the drawback with the foregoing approach is significantly increased saturation field (or decreased permeability), which increases rise time of the write field requiring higher coil current and thus greater power consumption with lower write accuracy. Saturation field is defined here as an external field required achieving approximately 95% saturation magnetization (an example of graph including saturation field responses, measured in Oersteds, is provided in FIG. 9). This is clearly undesirable for high data rate recording because, among other reasons, more heat is generated causing pole protrusion, which is likely to cause hard drive or disc drive failures.
Some of the issues associated with prior art methods and techniques vis-à-vis the design or manufacturing of the main pole is now discussed with reference to various figures.
In FIG. 1, an angled side view of a portion 10 of a perpendicular recording head is shown to include a main pole 12, a coil(s) 14 and a return pole 16. The main pole 12 and the return pole 16 form a U-shape around the coil 14. In operation, current flows through the coil 14 creating a magnetic field between the main pole 12 and the return pole 14. When programming or storing information onto a medium, magnetically-encoded information is written by creating a magnetic path from the main pole 12 to the medium and back to the return pole 16 to close the loop.
Generally, the narrower the main pole 12, the more bits can be written per unit area on to a medium. However, use of a narrow main pole has been known to introduce large remnant field, which causes undesired erasure of valid data stored in the medium.
FIG. 2 shows an example of a narrow main pole at 18. The reason for the large remnant field is, in large part, the presence of only a single domain at 20. Domains are magnetic orientations. In FIG. 3 however, due to the presence of a closed domain structure as a result of a wide main pole, shown at 21, the remnant field is small. That is, multiple domains, shown at 22-40, cancel each other thus leaving a small remnant field effect. It should be noted that the orientations or domains shown in FIGS. 2 and 3 are present in the absence of any writing or current being applied to the coil 14. In high density recorders, such as perpendicular recorders or heads, a narrow main pole 12 is preferable, such as that shown in FIG. 2, however, as earlier stated, the problem then becomes undesirable erasure due to the presence of large remnant field. Remnant field is generally the effects of previous magnetization. Thus, when programming or writing is performed and the head is shut off or not being employed for storage, the effects of remnant field or a residual field preside.
FIG. 4 shows a graph of the effects of a narrow main pole width on remnant field. The horizontal axis shows a measure of the main or write pole's width in nano meters microns (nm) and the vertical axis shows a measurement of the remnant field in Oersteds (Qe). As shown at 50, a main pole width of 150 nm results in remnant field of 1000 Oe while, as shown at 52, a main pole width of about 60 nm results in remnant field of 3000 Oe. Accordingly, as the width of the main pole is narrowed, which is desirable in high density recorders, the remnant field effects increase. The graph of FIG. 4 is measurements for a disc drive that has a track width of 60 to 200 nm, a write head that has a throat height of 200 nm, a main pole that has a Bs of 2.0 Tesla, a head-to-medium measurement of 15 nm, a medium thickness of 20 nm and a separation layer of 5 nm.
FIG. 5 shows, in graphical form, the undesirable effects of the residual effect and thus, instability of the main pole or write performance. The graph in FIG. 5 shows a horizontal axis that represents the number of read or write tests and the vertical axis represents a normalized output. The graph results from a test performed on the recording head where data or information is first written, such as 5 or 6 bits, and next, writing is stopped so that no current is flowing through the coil of the write head and the head is allowed to fly on top of the medium or disc (with no driving current). If no remnant field existed, the graph would show a bar across the horizontal line at 54 with a normalized output of 1.0 for the entire read and/or write tests. Instead, undesirable erasures result due to the presence of remnant field, as shown, for example, at 56-64.
FIG. 6 shows yet another graph of the effects of write instability as a function of the main pole width. The horizontal axis of the graph of FIG. 6 represents the magnetic write width in nm, which is proportional to the width of the main pole or write width and the vertical axis of the graph of FIG. 6 represents the change in output in percentage. The change in output is the change in the stored data or information and is calculated by the maximum value of an encoded value minus the minimum value thereof divided by the maximum value. For example, as the width is narrowed or becomes close to 150 nm, the percentage of change becomes approximately 80% or a near-complete loss of data. An output variation of 20% is generally regarded as undesirable or unacceptable because the error rate increases drastically.
FIGS. 7 and 8 show existing designs of main poles. FIG. 7 shows the main pole with only one material or lamination, namely, a CoFe alloy and a pole width of 60 nm and a throat height of 60 nm. The pole width is shown at 600 and the throat height is shown 620. As shown, the moments or domains 640, 66, 68 and 70 are scattered, pointing at different directions while no writing operation is in process. This, in effect, causes a high remnant field, in this case 1700 Ge while the main pole 12 of FIG. 8 has as its characteristic, a remnant field of only 200 Ge. The latter is due to the structure of the main pole 12 having three adjacent laminations, resulting in a multilayered pole. In the example of FIG. 8, three laminations or layers are employed, a non-magnetic layer 72, which serves as an underlayer to a magnetic layer 74, which, in turn, serves as an underlayer to another magnetic layer 76. The proximity of the magnetic and non-magnetic layers have the effect of canceling the moments when no current flows or no write operation is being performed because each of the layers have an opposite moment thereby canceling adjacent moments. For example, the moment 78 is opposite to that of the moment 80 and the latter is adjacent to the moment 82. Thus, the orientations of the moments of FIG. 8 are each in a particular direction or opposite to each other, whereas, the orientations of the moments of FIG. 7 are undefined or scattered. The net effect of the foregoing structure is AFC of adjacent laminations or layers and a substantial reduction in remnant field. Multilayering of the magnetic and non-magnetic layers of the pole 12 of FIG. 8 causes coupling, in opposite direction, of these layers resulting in AFC. The remnant field of the main pole 12 of FIG. 8 is 200 Ge. An example of a magnetic layer is one that is a CoFe alloy and an example of a nonmagnetic layer is one that is chromium (Cr).
FIG. 9 shows a graph of the driving (or external) magnetic field (measure in Oersteds (Oe)) vs. normalized moment for the two types of films of FIGS. 7 and 8, i.e. one film being made of a single layer, such as CoFe, and another film being made of multilayers and having AFC affects. The curves at 84 represent an eight-layer film, such as that of FIG. 8 and the curves at 86 represent a single layer film, such as that of FIG. 7. The graph of FIG. 9 is commonly referred to as a BH loop and in essence, it represents the change of magnetic moment in the material in responding to the external magnetic field. In the case of the main pole, the external field is generated by electrical current in a coil. The moment at zero field corresponds to remnant moment when the write head is not writing on the media. When writing or erasing, it is desired that pole material reaches the saturated moment for best writability. The less current needed to reach saturation, the more desirable because increased current causing increased heat in the head resulting in increased mechanical failures and lower recording or writing speeds.
As shown at 84, the multilayered pole design results in additional current or a slower recording time than the single layered pole shown at 86. Higher current also may result in pole protrusion thereby damaging magnetic properties and reducing read/write performance and increasing mechanical failure. To rephrase, requirements for higher field to reach maximum magnetization cause higher coil current to saturate the pole for providing sufficient overwrite, thus, more heat is generated causing pole protrusion. Furthermore, lower write speeds are realized that may not be suitable for high data rate drives.
Therefore, the need arises for a main pole employed in perpendicular recorders or disc drives having characteristics of low remnant magnetization and low saturation field.